Autism is a devastating condition which takes a significant toll on patients, their families, and the national economy. There are no treatments for cognitive impairments (e.g., learning and memory deficits) that affect over 75% of autistic patients. In models of many autism-associated disorders and conditions with comorbidity for autism, such Fragile X, Rett, and Angelman Syndromes, there are significant abnormalities in dendritic spine morphology, which are associated with impairments in the stabilization of long-term potentiation (LTP), a synaptic mechanism of memory encoding. Together, these findings suggest that defects in the spine actin cytoskeleton may underlie cognitive deficits in autism-associated conditions of different origin. Spine abnormalities and LTP impairments are best characterized in the Fmr1-knockout (KO) mouse model of fragile X syndrome (FXS), a condition with high (~30%) comorbidity for autism. Specifically, these mutants exhibit defects in signaling through Rac GTPase, stabilization of activity-driven changes in spine filamentous (F) actin, and consolidation of LTP. Studies by the applicant have demonstrated that movement of cortactin, a spine protein which stabilizes actin network branch points and protects F-actin from degradation, via both actomyosin and microtubule systems, is impaired at Fmr1-KO spines following LTP-induction. This suggests that the F- actin stabilization deficits in KOs may reflect disturbances in signaling to cortactin. The proposed research will build on these findings to test the specific hypotheses that (a) abnormal cortactin serine phosphorylation and acetylation, which regulate the protein's subcellular movement, both originate from a single molecular impairment and contribute to the phenotype of impaired movement following LTP induction in KOs and that (b) learning (in vivo) activates these synaptic processes in WT but not KO mice. There are 3 specific aims. Aim 1 will test if basal levels or activation of synaptic Ras or PP2A are impaired in KOs (both of these targets influence the cortactin phosphorylation and acetylation paths). Aim 2 will test if signaling through MAPK and/or HDAC6, which contribute to cortactin serine phosphorylation and acetylation, are necessary for activity- induced cortactin translocation. hippocampus-dependent spatial learning activates synaptic signaling to cortactin in the WTs and if this signaling is attenuated or absent in Fmr1-KOs in vivo. Through interrogating synaptic mechanisms associated with impairments in F-actin stabilization and determining if these abnormalities are also present in the behaving animal, the proposed studies will contribute to our understanding of synaptic plasticity in both normal and FXS model mice and offer therapeutic targets for normalization of memory function in FXS and other autistic conditions. Finally, Aim 3 will test if hippocampus-dependent spatial learning activatessynaptic signaling to cortactin in the WTs and if this signaling is attenuated or absent in Fmr1-KOs in vivo. Through interrogating synaptic mechanisms associated with impairments in F-actin stabilization and determining if these abnormalities are also present in the behaving animal, the proposed studies will contribute to our understanding of synaptic plasticity in both normal and FXS model mice and offer therapeutic targets for normalization of memory function in FXS and other autistic conditions.